Inversion of electromagnetic data is a process commonly used to identify geologic structures of interest for geophysical prospecting. A source of incident electromagnetic energy is typically towed in water behind a vessel, along with one or more receivers to detect response electromagnetic energy resulting from interaction of the incident electromagnetic energy with the earth, water, and air. The data thus gathered is compared to a geophysical model of the survey area. The known parameters, such as frequency, amplitude, and phase of the electromagnetic energy, and geometry of the survey, are used to model response electromagnetic energy, and the model result is compared to the measured data to determine geophysical parameters of the survey area. The geophysical parameter of most interest is usually electrical resistivity. A map of electrical resistivity is frequently drawn from the final electrical resistivity data, and the map can then be used to identify potential resource deposits for recovery.
In performing such inversions, it is typically not possible to fully invert for all depths from the sea surface into the subsurface. The wide range of resistivities encountered leads to problems of non-uniqueness and general difficulty in accurately converging the problem. For this reason, the resistivity of the seawater is typically pre-constrained. A common technique is to take a few independent resistivity measurements in the water, average them, and assume the seawater resistivity is constant. This approach reduces precision of the resulting subsurface resistivity map because seawater resistivity, in reality, can vary widely. In these situations, the recorded electromagnetic fields are not explained well by the models, and relatively large residual errors remain from the inversion, increasing uncertainty in interpreting the results.
Typical methods of dealing with the residues involve adjusting the assumed seawater resistivity and repeating the inversion. Such trial and error processes are time and resource intensive, and the quality of the result depends directly on the quality of the adjustments made to the seawater resistivity assumptions. Moreover, assuming constant seawater resistivity forces actual seawater resistivity variation into the data as unremovable noise.
There is a need for inversion methods that achieve better results using less time and computing resources.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.